[LLVMdev] A cool use of LLVM at Apple: the OpenGL stack

[Chris Lattner]

[I just got official okay to mention this in public.  This was previously 
announced at Apple's WWDC conference last week.]

For those who are interested, Apple announced that they are using the LLVM 
optimizer and JIT within their Mac OS 10.5 'Leopard' OpenGL stack (which 
was distributed in beta form to WWDC attendees).

LLVM is used in two different ways, at runtime:

1. Runtime code specialization within the fixed-function vertex-processing
    pipeline.  Basically, the OpenGL pipeline has many parameters (is fog
    enabled? do vertices have texture info? etc) which rarely change:
    executing the fully branchy code swamps the branch predictors and
    performs poorly.  To solve this, the code is precompiled to LLVM .bc
    form, from which specializations of the code are made, optimized,
    and JIT compiled as they are needed at runtime.

2. OpenGL vertex shaders are small programs written using a family of
    programming langauges with highly domain-specific features (e.g. dot
    product, texture lookup, etc).  At runtime, the OpenGL stack translates
    vertex programs into LLVM form, runs LLVM optimizer passes and then JIT
    compiles the code.

Both of these approaches make heavy use of manually vectorized code using 
SSE/Altivec intrinsics, and they use the LLVM x86-32/x86-64/ppc/ppc64 
targets.  LLVM replaces existing special purpose JIT compilers built by 
the OpenGL team.

LLVM is currently used when hardware support is disabled or when the 
current hardware does not support a feature requested by the user app. 
This happens most often on low-end graphics chips (e.g. integrated 
graphics), but can happen even with the high-end graphics when advanced 
capabilities are used.

Like any good compiler, the only impact that LLVM has on the OpenGL stack 
is better performance (there are no user-visible knobs).  However, if you 
sample a program using shark, you will occasionally see LLVM methods in 
the stack traces. :)

-Chris

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